1. Technical Field
The present invention relates to cladding pumping fiber lasers, and in particular relates to the fiber cladding geometry structure and pumping laser coupling method for construction of high-efficiency and high-power fiber lasers.
2. Background Art
The development of diode pumped fiber lasers has been rather successful recently. The scaling of various physical effects has greatly benefited this development. Diode lasers can provide concentrated pumping energy and thus enhance the efficiency of fiber lasers. The long thin geometry of fiber also makes heat removal much easier than in bulk solid state lasers. In end-pumped fiber lasers, a large outer cladding is used in cladding pumping. Pump light, often piped through fibers from pump lasers, enters the outer core, where it is confined so that it passes through the inner core, exciting the laser species. Stimulated emission from the laser species remains in the inner core. By converting the low brightness beam from the pump diode bar into a tighter beam, pumping a fiber laser can multiply brightness by a factor of more than 1000. By using such configuration, Polaroid, for example, reported a diode cladding pumping fiber laser reaching 35 W (M. Muedel et al CLEO '97, Postdeadline Paper CPD30, Baltimore, Md., 1997).
Currently, a typical fiber laser device includes a tens-of-meters double clad silica fiber with a small diameter and small NA core doped with active species, centered within a much larger inner cladding, surrounded by a soft low index fluoropolymer providing an acceptance NA of 0.45 for pump radiation. Pumping laser beams from laser diodes are coupled into the fiber inner cladding through the dichroic end mirror. (HR laser, HT pump). Among other things, the cladding pumping geometry and the coupling efficiency of pumping laser diode array are the main factors under intense research. Proper geometry is essential for increasing the efficiency of cladding pumping. Good method of coupling will allow more power to be injected into the fiber laser, which in turn also increase the efficiency of a fiber laser.
There are many patents dealing with cladding pumping. U.S. Pat. Nos. 5,533,163, 4,829,529, 4,815,079 disclose various cladding cross-sectional geometric shapes such as circle, rectangle, convex polygon (triangle, rhombus, hexagon). These cladding boundary (CB) shapes, however, have certain obvious disadvantages. The main disadvantage is the presence of local modes, and the pumping beam localized in such modes can not enter the core.
When skew ray is reflected on an arbitrary cylindrical surface, the projections of incident ray and reflected ray on the principal cross section are like the light ray reflected in this plane. Therefore, we can use the behavior of light beams in the principal cross section of cylindrical surface to determine local modes of fiber cladding.
The boundary shapes of fiber cladding geometry include circular, rectangle, right triangle, isosceles triangle, and rhombus. As a comparison with the current invention, the local modes in fiber cladding with different boundary shape are summarized as follows.
FIG. 1 shows a schematic illustration of a light beam path in prior art circular fiber cladding with a circular cladding boundary CB. Because the sag of ray keeps constant in multiple reflection, the light beams LB.sub.0 initial at the outer region can not reach central region through multiple reflection. Therefore, the center position is not a good location for core A, and the core must be close to the boundary as position B although center position is usually more preferred due to the structures of fiber connections. FIG. 2 is a schematic illustration showing the local modes in prior art rectangular fiber cladding with a boundary CB. There are two types of local modes in the rectangular cladding. One is the light beam perpendicular to the boundary (LB.sub.1, and LB.sub.2), the other one is the light beam parallel to the line joining two corners (LB.sub.3). These light beams form different close cyclic loops in the rectangular boundary. Besides these two fiber cladding geometry shapes, there are also right triangle, isosceles triangle and rhombus. For right triangle case, the light beams perpendicular to the hypotenuse form different close loops in the right triangle boundary as shown in FIG. 3 (LB.sub.4 and LB.sub.5). In isosceles triangle fiber cladding, there are two types of local modes as shown in FIG. 4. One is the light beam perpendicular to the leg (LB.sub.6), the other type is the light beam parallel to the base (LB.sub.7). Rhombus cladding behaves like two isosceles triangles. The local modes in rhombus cladding are the same as in isosceles triangle as shown in FIG. 5.
If a fiber cladding has some local modes and the core is not in the region of the local modes, the pumping light beam of local modes can not reach core and the pumping efficiency will decline.
In order to avoid the presence of local modes, sometimes bending in the fiber structure is suggested to provide perturbation in the modes propagating in the multi-mode cladding. However, the effect of bending on perturbation is not clear, and can not be accurately predicted. It will be much more favorable to find new cladding geometry structures so that local modes can not be generated, or at least the local modes are limited near the core area and the pumping beam can easily enter the core. In this way, the efficiency can be increased and the length of fiber lasers can be reduced.
Currently, the leading company in fiber laser research and manufacturing is Polaroid. One fiber laser of Polaroid was reported to have high efficiency (about 65%), but this efficiency is the ratio of pumping laser power entered the optical fiber and the output power of the fiber laser. Therefore, the efficiency of coupling or power injection is not considered. In this Polaroid fiber laser, three fiber-coupled SDL P6 diodes are spatially combined and de-magnified into a rectangular cladding. The slop efficiency of the diode lasers is only 0.5 W/A, while the efficiency of a non-fiber coupled diode laser is much higher, 1.25 W/A. Therefore the total electric efficiency of this Polaroid fiber laser is not high. In addition, due to the limitation of the cladding geometry, tens of meters of fiber must be used in this state-of-the-art system. Furthermore, since it is difficult to couple more beams into an optical fiber with the coupling method used in the Polaroid systems, it is difficult to develop a fiber laser with even higher power, such as 1000 W CW. It is therefore necessary to find new methods to couple high power into optical fibers and improve the pumping efficiency.
A typical high-power laser diode array (LDA) has an a broad area light emitting aperture (1 cm.times.1 .mu.m) comprising light emitting elements which are multiple spaced apart segments. In one typical commercial LDA product, for example, each segment has a width less than 200 .mu.m, and may be divided into 20 sub-segments. Each sub-segment has an aperture width of 3-6 .mu.m, and emits about 30 mW-60 mW. The effective aperture size in the transverse direction perpendicular to the plane of laser active region (the fast axis) is about 1 .mu.m. Typical fast axis divergence is 30-40 degree and slow axis divergence is 10-15 degree. A typical high-power LDA can deliver 20 W of laser power. Those more powerful can deliver 40 W or 60 W with this geometry. Because of the broad geometry of LDA, it has been always a challenge to couple or inject high power (such as 4000W) into a fiber cladding aperture (such as an aperture of 200 .mu.m.times.500 .mu.m, NA 0.45).
In order to send more power into a cladding fiber, many efforts have been made to concentrate light from diode laser arrays. There are a number of patents dealing with concentrating multiple emitter laser diode beams, such as U.S. Pat. Nos. 5,802,092, 5,793,783, 5,790,310, 5,594,752, 5,579,422, 5,568,577, 5,333,077, 5,185,758, 5,139,609, and 4,428,647.
In U.S. Pat. No. 4,428,647, Spragne et al disclose systems in which each laser emitter of a diode laser array has its own lens mount adjacent to it in the space between the laser array and objective lens of the system. The purpose of the lens array is to change the angle of divergence of light beams leaving the emitting surface of the laser array at the slow axis so that the light beam can be collected efficiently by the objective lens. In U.S. Pat. No. 5,185,758 and in an earlier article (Optical Letters. Vol. 14, p.1087, 1989) Fan et al describe a method for scaling a pumped medium to higher power with multiple light source. The output beam of each light source is substantially collimated by respective collimating optics, and the beams of sources are substantially parallel to each other after collimation. An optical system is provided to focus the collimated and parallel beams. The methods described in U.S. Pat. No. 5,802,092, 5,793,783, 5,790,310, 5,594,752, 5,579,422, 5,568,577, 5,333,077, and 5,139,609 are similar to the methods mentioned above. However, since lens arrays can only collimate the beam from the diode array to a limited extent, obvious divergence still exists. Because of the beam divergence, laser diode arrays must be close to an optical fiber so that the beam spot is small enough to achieve effective coupling. When multiple laser diode arrays are combined, the dimension of beam spot on the fiber aperture plane becomes larger due to the increased distance between laser diode arrays and the fiber aperture. As a result, these methods can not efficiently combine the beams from a plurality of diode laser arrays in to an optical fiber. For example, with these methods, it is impossible to effectively couple the beams from 200 pieces of 20W diode laser bars into a fiber to make a high-power fiber laser.
A need therefore exists in the art for a method to make high-efficiency, high-power fiber lasers, while high-efficiency cladding pumping geometry and effective LDA-fiber coupling are the most desired.